1. Field of the Invention
This invention relates to a system for measuring the inflation pressure of a tire on a vehicle wheel and subsequently transmitting measured real time pressure values to the vehicle, which system may also be used to transmit measured temperature values to the vehicle.
2. Description of the Prior Art
It is an increasing requirement in modern day passenger carrying aircraft that the pressure of each tire on the aircraft undercarriage be monitored continuously during taxiing, takeoff, flight and landing.
The operating temperature range, typically -55.degree. C. to +160.degree. C., the accuracy, typically .+-.2% of reading, and size constraints required of some systems are such that they cannot be met by a system using an uncompensated sensor. Such uncompensated sensors have the disadvantage that their output is non-linear and varies with temperature.
U.S. Pat. No. 4,845,649 discloses a sensor unit for measuring inflation pressure of a vehicle tire which includes a pressure sensor, an analogue to digital converter, a logic circuit and a programmable read only memory (PROM) disposed on a chip which consists of an electrically nonconductive material. Before being installed on a vehicle wheel, the unit is mounted on a test stand for calibration of the pressure sensor and correcting values for linearising sensor outputs are determined and stored in the PROM. A disadvantage of the disclosed arrangement is that in operation, with the sensor unit installed on a vehicle wheel, both pressure sensor measurement values and correction data from the PROM are transmitted to a microprocessor on the vehicle for processing to obtain a corrected pressure signal. This requirement for transmission of two data signals between the wheel and the vehicle introduces two possible sources of error. Furthermore, although this reference proposes that the temperature fluctuations may be taken into account during calibration of the pressure sensor, there is no disclosure of temperature measurement during operation of the sensor unit. Thus, the sensor arrangement disclosed in U.S. Pat. No. 4,845,649 corrects the measured pressure value and outputs a pressure reading that is compensated back to a normal or cold tire state.